A system for in situ replacement of cutting means for a ground drill is described in Applicant's International application no. PCT/AU94/00322 (WO 94/29567), the contents of which are incorporated herein by way of reference.
The system in WO 94/29567 comprises a drive sub which is adapted for connection to a lower end of a core barrel attached to a drill pipe; a tool for installing and retracting drill bit segments from the drive sub; and, an insert or bit locking sleeve for selectively locking the bit segments into seats provided about the inner circumferential surface of an end of the drive sub and subsequently releasing the bit segments for those seats. The tool includes a main body portion and a sleeve slidably mounted thereon. Installation latch dogs provided in the tool extend from apertures or slots cut in the sleeve so as to engage the bit locking sleeve and force it into an installation position in which it locks the bit segments in a cutting position about the drive sub. The tool further includes retrieval latch dogs which can extend from different slots provided in the sleeve for engaging the bit locking sleeve and pulling it upwardly into a retrieval position in which the bit segments can be retrieved from the drive sub.
A slidable cradle extends from a lower end of the tool for carrying the bit segments to and from the drive sub. When installing the bit segments, the cradle is extended from the lower end or head of the tool against the bias of a spring. Bit segments are held by rubber bands about the cradle with one end abutting a stop provided at one of the cradle and an opposite end bearing against the head of the tool. When the tool is lowered into the ground drill (comprising the combination of the drill tube, core barrel and drive sub) and reaches a predetermined position within the drive sub (that being the point of engagement with the bit locking sleeve), the sleeve is caused to move relative to the main body of the tool which in turn releases a set of pins holding the spring about the cradle in compression. This fires the cradle so that the spring is able to expand, retracting the cradle into the main body of the tool which causes an upper end of the bit segment to slide along the head of the tool so as to extend laterally of the outer periphery of the tool. The bit locking sleeve is simultaneously pushed by the tool so as to catch the ends of and move inside the drill bit segments thereby expanding the drill bit segments to the inner diameter of the drive sub and locking the drill bit segments in the cutting position.
When lowering the tool into the ground drill the tool is initially placed within a transport sleeve which acts to compress the installation latch dogs to prevent catching on internal surfaces of the drill tube prior to entering a core barrel and the drive sub. A landing ring is provided between the core barrel and drill tube of a diameter which prevents further progress of the transport sleeve but allows the tool to pass therethrough. The transport sleeve sits on the landing ring and, after installation or retrieval of the cutting means again carries the tool once pulled from beneath the landing ring to the surface.
Field trials of the above system have proved very successful. Nevertheless, it is thought that there is a potential for various problems to arise under extreme operational conditions.
In the system of WO 94/2956, retrieval latch dogs are used in order to engage the bit locking sleeve for pulling it from the installation position to the retrieval position. This allows retrieval of the cutting means with the withdrawal of the tool from the ground drill. However, in order to allow the tool to be withdrawn, the retrieval latch dogs must be disengaged from the bit locking sleeve. In the above system, this is achieved by the provision of clips retained within the drive sub. The clips which function to hold the bit locking sleeve in the retrieval position when shifted thereto by the tool, are provided with tapered surfaces which engage the retrieval latch dogs lifting them out of contact with the inner surface of the drive sub and forcing them toward the inside of the tool. However, this does not fully retract portions of a latching face of the retrieval latch dogs into the tool. In order to allow the retrieval latching dogs to pass back through the landing ring, a bevel is formed on the latching face which upon engagement with the landing ring when the tool is being pulled upwardly further compresses the retrieval latch dogs inwardly to allow retraction of the tool through the landing ring.
There is thought to be a possibility that the clips used in the system of WO 94/29567 may be dislodged or damaged and not function so as to compress the retrieval latch dogs. Further, the manufacture of the clips and fitting to the drive sub increases the overall cost in the system.
It is a general object of the present invention to provide an alternate and more reliable system for retracting a latching system of a tool or other apparatus.
It is a more particular object of the present invention to provide a more reliable retraction system for the retrieval latch dogs of a down hole tool used in a system for in situ replacement of cutting means for a ground drill.